Traffic lights, traffic signals, or stoplights – also known as robots in South Africa, Zambia, and Namibia – are signalling devices positioned at road intersections, pedestrian crossings, and other locations to control the flow of traffic.
Traffic lights usually consist of three signals, conveying meaningful information to road users through colors and symbols, including arrows and bicycle symbols. The usual traffic light colors are red to stop traffic, amber to signal a change, and green to allow traffic to proceed. These are arranged vertically or horizontally in that order. Although this is internationally standardized, variations in traffic light sequences and laws exist on national and local scales.see Variations in traffic light operation
Traffic lights were first introduced in December 1868 on Parliament Square in London to reduce the need for police officers to control traffic. Since then, electricity and computerized control have advanced traffic light technology and increased intersection capacity. The system is also used for other purposes, including the control of pedestrian movements, variable lane control (such as tidal flow systems or smart motorways), and railway level crossings.
History
In December 1868, the first traffic signals showing a red or green light at night were installed outside the Houses of Parliament in London. They were invented by John Peake Knight. A police constable raised or lowered the semaphore arms and, at night, operated a lever to control the lights which drivers and pedestrians saw. This system exploded on 2 January 1869 and was taken down. This early traffic signal led to other parts of the world implementing similar traffic signal systems. In the first two decades of the 20th century, semaphore traffic signals like the one in London were in use all over the United States. These traffic signals were controlled by a traffic officer who adjusted the signals to direct traffic.
In 1912, the first electric traffic light was developed by Lester Wire, a policeman in Salt Lake City, Utah. In August 1914, the American Traffic Signal Company installed it on the corner of East 105th Street and Euclid Avenue in Cleveland, Ohio. In 1920, the first four-way, three-color traffic light was created by William Potts in Detroit, Michigan. His design was the first to include an amber 'caution' light along with red and green lights. Potts was Superintendent of Signals for the Police Department of Detroit. In 1921, he installed automatic four-way, three-color traffic lights in 15 towers across Detroit.
By 1922, traffic towers were being controlled by automatic timers more widely. The main advantage of using the timer was that it saved cities money by replacing traffic officers. New York City was able to reassign all but 500 of its 6,000 officers working on the traffic squad, saving the city $12,500,000. In 1923, Garrett Morgan patented a design of a manually operated three-way traffic light with moving arms.
The control of traffic lights changed with the rise of computers in America in the 1950s. One of the best historical examples of computerized control of lights was in Denver, Colorado, in 1952. In 1967, Toronto, Canada, was the first to use more advanced computers better suited to vehicle detection. The computers maintained control over 159 signals in Toronto through telephone lines.
Vehicular signals
A set of lights, known as a signal head,Traffic Signs Manual – Chapter 6: Traffic Control. Department for Transport, Department for Infrastructure (Northern Ireland), Transport Scotland and Welsh Government. Accessed: 18 December 2021. may have one, two, three, or more aspects. The most common signal type has three aspects facing the oncoming traffic: red on top, amber ("yellow" in North America, "orange" in Australia) below, and green below that. Additional aspects may be fitted to the signal, usually to indicate specific restrictions or filter movements.
Meanings of signals The 1968 Vienna Convention on Road Signs and Signals Chapter III provides international standards for the setup of traffic signal operations. Not all states have ratified the convention. A three-colour signal head should have three non-flashing red, amber, and green lights. They should be arranged either vertically, with red on top, or horizontally, with red opposite the traffic flow. A two-colour signal head may be used in temporary operation and consists of red and green non-flashing lights. In both cases, all lights should be circular or arrow-shaped. Permissible signals for regulating vehicle traffic (other than public transport vehicles) are outlined in Article 23: +Standard meanings for traffic lights internationally (Vienna Convention, Article 23)Light typeMeaningNon-flashingRedTraffic may not proceed beyond the stop line, or otherwise enter the intersection.Red/AmberThe signal is about to change, but the red light rule continues to apply.AmberTraffic may not pass the stop line or enter the intersection, unless it cannot safely stop when the light shows.GreenTraffic may proceed, unless it would not clear the intersection before the next change of phase.FlashingRedTraffic must not pass the stop line at a level crossing, approach to swing bridge or ferry landing stages, emergency vehicles entering the road, or on the approach to low-flying aircraft. In addition, to attract attention in these locations, it is also equipped with additional alarms and voice prompts.AmberTraffic may proceed, but with caution. Green arrows are added to signals to indicate that traffic can proceed in a particular direction while the main lights for that approach are red, or that traffic can only proceed in one particular direction. Alternatively, when combined with another green signal, they may indicate that turning traffic has priority over oncoming traffic (known as a "filter arrow"). Flashing amber arrows typically indicate that road users must give way (to other drivers and pedestrians) before moving in the direction of the arrow. These are used because they are safer, cause less delay, and are more flexible. Flashing amber arrows will normally be located below the solid amber.
Green arrows Arrow aspects may be used to permit certain movements or convey other messages to road users. A green arrow may display to require drivers to turn in a particular direction only or to allow drivers to continue in a particular direction when the signal is red. Generally, a green phase is illuminated at the beginning of the green phase (a "leading turn") or at the end of the green phase (a "lagging turn"). An 'indicative arrow' may be displayed alongside a green light. This indicates to drivers that oncoming traffic is stopped, so they do not need to give way to it when turning across it. As right-turning traffic (left-side drive) or left-turning traffic (right-side drive) does not normally have priority, this arrow is used to allow turning traffic to clear before the next phase begins.
Some variations of this setup exist. One version is a horizontal bar with five lights – the green and amber arrows are located between the standard green and amber lights. A vertical five-light bar holds the arrows underneath the standard green light. In this arrangement, the amber arrow is sometimes omitted, leaving only the green arrow below the steady green light, or possibly an LED-based device capable of showing both green and amber arrows within a single lamp housing.
A third type is known as a "doghouse" or "cluster head" – a vertical column with the two normal lights is on the right side of the signal, a vertical column with the two arrows is located on the left, and the normal red signal is in the middle above the two columns. Cluster signals in Australia and New Zealand use six signals, the sixth being a red arrow that can operate separately from the standard red light.
In a fourth type, sometimes seen at intersections in Ontario and Quebec, Canada, there is no dedicated left-turn lamp per se. Accessed: 25 March 2014 Instead, the normal green lamp flashes rapidly, indicating permission to go straight as well as make a left turn in front of opposing traffic, which is being held by a steady red lamp. This "advance green", or flashing green, can be somewhat startling and confusing to drivers not familiar with this system. This can confuse visitors to British Columbia, where a flashing green signal denotes a pedestrian-controlled crosswalk. For this reason, Ontario is phasing out the use of flashing green signals and instead replacing them with arrows. Accessed: 25 March 2014
Countdown lights Popular in Vietnam and China, countdown lights are additional lights installed adjacent to or within the main signal lights. The countdown light is displayed by a countdown number with different colours, usually red, yellow, and green, matching the colour of the light on. When the light counts to "0" (or 1), the main light colour immediately changes.
Countdown lights may have zeros in the tens place or none; some may flash when getting ready to zero. Yellow lights can have countdown lights, but the most lights do not. Usually the countdown light has 2 digits, in case the time of the main light (usually the red light, rarely the green light) is longer than 100 seconds, depending on the type of light, the following possibilities may occur:
The lights have not counted down, when 99 seconds are left, start counting. During standby, the light may display "99", "00", "--", or be off. The last two digits count light of the timeout (the counter light is 15 while the time is 115 seconds, some types of lights count as "-9" or "9-" when the time is 109 seconds) The tens digit on the display becomes a letter. Displaying A0 for 100 seconds, B0 for 110 seconds, so forth. Display of only the last two digits, with a flashing indicator to show it is more than 100.
Countdown lights are also used for both vehicular and pedestrian signals in the Philippines. However, since 2025, the use of countdown timers has been phased out in Metro Manila by the Metropolitan Manila Development Authority in favor of a volume-based traffic signalling system. As a result, instead of a countdown timer to the next phase, green signals will blink five times before turning yellow.
Issue about the yellow light dilemma zone in South Korea In South Korea, the yellow (amber) light dilemma zone is not legally recognised. In other words, when the amber light is on, traffic may not pass the stop line or enter the intersection, even if the traffic cannot safely stop when the light shows.
In May 2024, this was reaffirmed by the Supreme Court of Korea, for a case where the driver was speeding at 62 km/h in a street limited up to 40 km/h, % higher than the allowed speed.
Critics in South Korea say this is unrealistic and unreasonable. This can cause multiple collisions due to sudden braking.
In 2016, when the speed limit was up to 60 km/h, proposed alternatives to this type of collision included only roundabouts, increased speed compliance, and reduced-speed practice; elderly zones were also proposed as solutions.
Yellow trap Without an all-red phase, cross-turning traffic may be caught in a 'yellow trap'. When the signal turns yellow (amber), a turning driver may assume oncoming traffic will stop, and a crash may result. For this reason, the US bans sequences that may cause a yellow trap. This can also happen when emergency vehicles or railroads preempt normal signal operation. In the United States, signs reading "Oncoming traffic has extended green" or "Oncoming traffic may have extended green" must be posted at intersections where the "yellow trap" condition exists.
Variations
United States